The present invention relates to a flow control honeycomb structure for producing turbulent flows in an air-fuel mixture flowing through an air-fuel mixture intake passage coupled to an internal combustion engine in order to promote vaporization of fuel and mixture thereof with air for an improved operation efficiency of the engine, and a method of making such a flow control honeycomb structure.
Carburetors associated with internal combustion engines have venturis for drawing in and vaporizing fuel with air flowing through the venturi throat. When the engine is to be started at a low temperature, however, the fuel is not easily vaporized in the carburetor since it is also at a low temperature and a high viscosity whereby it tends to remain in large droplets. Because such large fuel droplets cannot be well mixed with air, an air-fuel mixture of desired uniform density cannot be produced.
One prior proposal to avoid such a problem has been to provide a generally conical flow control body disposed in an air-fuel mixture intake passage downstream of a carburetor, as disclosed in U.S. Pat. No. 3,998,195. The flow control body has many small passageways for an air-fuel mixture to pass therethrough to produce turbulent flow in the air-fuel mixture for promoting vaporization of fuel. The air-fuel mixture as it flows through the passageways is supposed to be disturbed and mixed across the entire cross-sectional area of the flow of the air-fuel mixture. The disclosed passageways are defined by long small holes of constant cross section, which however tend to impose large resistance on the air-fuel mixture flow therethrough. Further, the conical flow control body with such many small holes is difficult to manufacture.
It has been found that a honeycomb flow control structure positioned in an air-fuel mixture intake passage can also promote fuel vaporization for a better air-fuel mixture through disturbances or turbulences produced on the downstream side of the flow control structure.
Products of honeycomb structure are generally manufactured by combining corrugated panels. The diecasting process is also widely used to make plates of honeycomb structure that cannot easily be fabricated from such corrugated panels. Honeycomb-shaped plates for use in the air-fuel mixture passages of engines for producing turbulent flows in the air-fuel mixture are of relatively small dimensions and should preferably be cast for the purpose of mass production. Such honeycomb-shaped turbulence plates are structurally independent of the cross-sectional shapes of small passages defined therein, and hence may be of a cross-sectional shape that is selected from the standpoint of easy preparation of casing molds.
FIG. 10 of the accompanying drawings illustrates a conventional mold assembly for die-casting a honeycomb-shaped turbulence plate. The mold assembly comprises an upper mold U and a lower mold L which are combined together with mold cavities S defined therebetween for introducing a molten material therein. When a honeycomb-shaped turbulence plate is die-cast by the illustrated mold assembly, burrs or ridges are produced on the die-cast plate by the space between the upper and lower molds U, L and project into the small passageways in the turbulence plate. It would be very difficult to remove such burrs and ridges from the small passageways. The small passageways with burrs and ridges remaining therein present undesirably large resistance to the flow of an air-fuel mixture through the turbulence plate placed in the air-fuel mixture intake passage.
Conventional honeycomb-shaped turbulence plates are generally circular in cross section which are complementary to the cross-sectional shape of air-fuel intake passages in which they are to be installed. The plates have crossing partitions therein defining small passageways of substantially square cross section as shown in FIGS. 11 and 12 of the accompanying drawings which schematically illustrate two representative conventional honeycomb-shaped turbulence plates. The honeycomb-shaped turbulence plate shown in FIG. 11 comprises an overall circular-shaped passage C1 divided by crossing partitions r into passageways s of substantially square cross section and relatively small passageways a of substantially triangular cross section positioned at the periphery of the circular passage C1. The honeycomb-shaped turbulence plate shown in FIG. 12 also is circular and has a circular passage C2 divided by crossing partitions r into passageways s of substantially square cross section and relatively large passageways b of substantially triangular cross section positioned at the periphery of the circular passage C2. The triangular passageways a are much larger than the square passageways s (FIG. 11), and the triangular passageways b are much larger than the square passageways s (FIG. 12). Therefore, the air-fuel mixture flowing through the small triangular passageways a are subjected to too large resistance, and the air-fuel mixture flowing through the triangular passageways b are not subjected to adequate turbulence.
When such a honeycomb-shaped turbulence plate is die-cast in a mold, it must be removed from the mold by ejector pins. Accordingly, the die-cast plate is required to have portions for engagement by the ejector pins, which portions must have considerably larger areas as compared with the normal thickness of each partition in the honeycomb structure. Thus, those passageways which are positioned around the pin engaging portions are smaller in cross section than the other passageways. Since the pin engaging portions are normally located in the peripheral region of the turbulence plate, the peripheral region of the turbulence plate, especially as shown in FIG. 11, tends to impose large resistance to the flow of the air-fuel mixture through the turbulence plate.
When the throttle valve in the carburetor is opened to a small extent as during the starting of the engine, the air-fuel mixture tends to flow along the inner peripheral surface of the air-fuel intake passage. Thus, if the resistance to flow in the peripheral region of the turbulence plate is too large or, conversely, the passageways in the peripheral region of the turbulence plate are too large in cross-sectional area to produce adequate turbulence, the turbulence plate is not sufficiently effective in producing turbulent flows when the air and fuel are required to be well mixed. The fuel is apt to flow as a liquid film down the inner peripheral surface of the air-fuel intake passage. Consequently, the turbulence plate, particularly its peripheral region, should be effective enough to get such liquid fuel well atomized.
Another known proposal for better fuel atomization at a cold engine start is disclosed in Japanese Laid Open Utility Model Publication No. 55(1980)-83247. The disclosed arrangement includes an electric heater composed of a number of tubular bodies and disposed in an air-fuel mixture intake passage for heating an air-fuel mixture passing through the tubular bodies for promoting fuel vaporization. The tubular bodies have wall surfaces inclined toward the center of the riser of an intake manifold so that the fuel will be brought into better contact with the wall surfaces and the air-fuel mixture flowing from the tubular bodies will impinge on each other at the center of the riser. Two of the problems with the electric heater are that the electric load is increased when the engine is started, and a certain period of time is required to raise the temperature of the electric heater adequately for heating the air-fuel mixture. The air-fuel mixtures flowing from the tubular bodies are agitated by their mutual impingement, and the air and fuel mixing can be improved since the fuel is broken up into finer particles by such mutual impingement. However, since the air-fuel mixture is agitated only at the center of the riser, air and fuel cannot be well mixed completely.
Other turbulence plate and heater type devices for improving fuel vaporization are disclosed in U.S. Pat. Nos. 3,459,162; 3,826,235; 3,966,430; and 4,108,125, for example.